Apparatus for supercritical cleaning

ABSTRACT

An apparatus for precision cleaning with carbon dioxide includes a pressure vessel having a removable cleaning drum with a sanitized work zone, a separator for regenerating spent carbon dioxide, and pressure and temperature monitoring and control devices to maintain the carbon dioxide in a supercritical state while it is circulated in contact with a part to be cleaned. The sanitized work zone is defined by an impermeable drum body having removable entry and exit filters to ensure that circulating cleaning fluid does not redeposit contaminants on the parts. The separator includes distillation means. The pressure vessel and separator form part of an overall system, including a storage vessel for carbon dioxide, a pump, a preheater, a let-down valve in communication with the pressure vessel outlet, a condenser and a bypass. 
     The carbon dioxide is maintained in liquid form in the storage vessel, raised above its critical temperature by the preheater, pumped into the pressure vessel to achieve critical pressure, circulated through the work zone for a predetermined period of time, and removed through the let-down valve. The separator regenerates the carbon dioxide and carbon dioxide gas is then condensed and sent to the storage vessel for reuse.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This application is related to precision cleaning systems and, moreparticularly, apparatus for cleaning parts with supercritical fluids,with or without supplemental cleaning techniques.

2. Description of the Prior Art

Today's manufacturing and assembly industries require parts which have ahigh degree of cleanliness. These requirements have led to developmentof an independent area of technology known as "precision cleaning".Precision cleaning may be defined as cleaning a given part to a degreethat the level of foreign substances on the part meets a repeatablymeasurable standard. For example, parts which are to be chrome platedmust be cleaned to a contaminant level of 20 micrograms per squarecentimeter, or less. Disc drive components for computers must be cleanedto a level less than 5 micrograms per square centimeter, and wafersutilized in the electronics industry must be cleaned to a level lessthan 1 microgram per square centimeter. The various contaminants removedby precision cleaning include dissolvables, such as cutting fluid,particulates, such as diamond dust, and ionic bindings. Applications forprecision cleaning include the manufacture of pens, razors and computerchips as well as various electronics industry applications.

The problem with presently available precision cleaning systems is thatthey use chlorofluorocarbons (CFC's) which are considered to destroy theearth's ozone layer. A system which utilizes CFC's is disclosed in U.S.Pat. No. 4,443,269 to Capella, et al. ("Capella"). Capella discloses adecontamination method for radioactive tools utilizing a high pressurespray gun for spraying the contaminated tools with freon. The generalsolution is to utilize more benign cleaning solvents, such as carbondioxide. Carbon dioxide is particularly advantageous because it is anonpolar solvent so that cosolvents may be added for a high degree ofselectivity. It has been found that the cleaning capability of solventssuch as carbon dioxide is enhanced when the solvent is raised tosupercritical temperatures and pressures, or when supplemental cleaningtechniques are utilized, such as pressure pulsing and sonic treatment.

The general concept of cleaning with supercritical fluids is known inthe art. U.S. Pat. No. 5,013,366 to Jackson, et al. ("Jackson")discloses a cleaning process using phase shifting of dense phase gases.The solvent is shifted from its critical state to the liquid state andback by temperature adjustment while the solvent is in contact with thepart to be cleaned. The cleaning apparatus utilized in Jackson is shownin FIG. 6. However, Jackson discloses no internal filtration for thecleaning fluid so that cleaning fluid which has removed contaminantsfrom the part may redeposit the same on the part during circulationthrough the vessel.

Other cleaning systems utilizing circulated fluids, such as air, aredisclosed in U.S. Pat. Nos. 4,936,22; 4,832,753; 4,844,743; 4,576,792;and 4,290,821. However, none of these systems is directed tosupercritical cleaning. An apparatus for treating a workpiece atelevated temperatures and pressures is disclosed in U.S. Pat No.4,151,400 to Smith, Jr., et al. Additionally, an electric oven having aninternal circulation fan and heating elements is disclosed in U.S. Pat.No. 1,986,088 to Wild. Neither of these patents is directed to cleaning.

Finally, regarding cleaning fluid regeneration, Jackson schematicallydiscloses in FIG. 4 a separator 28 in communication with the outlet onthe cleaning vessel for recycling cleaning fluid. More detaileddisclosures for devices used to separate liquid and other substancesfrom gases may be seen in U.S. Pat. Nos. 4,879,004; 4,657,487;4,441,871; 3,997,303; and 3,063,259. However, none of these patents isdirected to a filter or separator for use in regenerating cleaningfluids for precision cleaning.

The apparatus used for precision cleaning is critical to success of thesystem. The apparatus must afford a high degree of temperature andpressure control and must be resistant to the high pressures required toachieve supercritical states. The apparatus must also be adaptable toremove particulates to varying degrees, especially when the apparatus isused in connection with parts testing. Importantly, the apparatus mustbe adapted to continuously circulate cleaning fluid in a closed system,yet avoid reapplication of contaminants onto the parts. Finally, theapparatus must provide uniform temperature and pressure throughout thecleaning vessel, while simultaneously maintaining different temperaturesand pressures in other components of the overall system. It is anadvantage according to this invention to satisfy all of the above statedrequirements.

SUMMARY OF THE INVENTION

Briefly, according to this invention, there is provided an apparatus forprecision cleaning, including a pressure vessel having a cleaning drumfor receiving a workpiece. The drum has an extended body, preferablyvertically oriented, with an entry filter and an exit filter thereon,and the drum body cooperates with the entry and exit filters to define asanitized work zone. The pressure vessel further defines an annularspace around the drum body. The cleaning drum is preferably removable.

The cleaning fluid is introduced through an inlet into the annular spaceand by the action of a positive circulation device is directed along thedrum body. The cleaning fluid then enters the work zone through theentry filter where it contacts the workpiece to remove particulates anddissolve contaminants thereon. The cleaning fluid then passes throughthe exit filter and recirculates to the annular space, with a portion ofthe cleaning fluid drawn from the work zone through an outlet when adownstream let-down valve is opened. A heating element in the vessel,along with the let-down valve may be controlled to maintain the cleaningfluid in a supercritical state. Alternatively, phase shifting of thecleaning fluid may be effected if desired.

Preferably, the filters are interchangeable to vary the mesh size ofparticulates passable therethrough. Also, the apparatus preferablyincludes a mesh basket, or other suitable device, in the work zone forsupporting the workpiece. The drum body should be impermeable to thecleaning fluid to avoid intermixing spent and regenerated cleaningfluid.

Preferably, a separator is provided in combination with the pressurevessel to remove particulates and mist from the cleaning fluid. Theseparator comprises an elongated body with a concentric feed tube. Thefeed tube has a plurality of longitudinally spaced apertures adjacentits distal end, and first and second filters are carried on the feedtube. An outlet is adjacent the upper end of the body in communicationwith the second filter. A collection chamber and drain are located atthe lower end of the body.

The spent cleaning fluid passes through the feed tube and is introducedto the filters through the spaced apertures. The filtered cleaning fluidthen exits the separator through the outlet, with the coalescedparticulates and mist falling from the filters to the collection chamberto be removed through the drain.

Because the cleaning fluid is typically introduced to the separator in acombined liquid/vapor state, the separator preferably includes an innercapsule enclosing the first filter and having an elongated outlet at itslower end. Spent cleaning fluid passes through the first filter andenters the capsule, wherein coalesced particulates and contaminants,along with liquefied cleaning fluid, passes downward through the outletinto the collection chamber. The liquid is heated to distillcontaminants therefrom. Vaporized cleaning fluid comes out of thesolution with the contaminants and passes upward around the capsule andthrough the second filter to the separator outlet. Distilledcontaminants are periodically evacuated through the drain. A levelindicating device maintains a predetermined amount of liquid in thecollection chamber.

It has been found that this separator arrangement, in line with thecleaning vessel, provides a high degree of cleanliness in the fluiditself, despite the continued recirculation of the fluid into contactwith the contaminated parts. This avoids redeposition of contaminantsonto the parts.

Finally, the invention includes a computer controlled system forcleaning a workpiece with a supercritical cleaning fluid. The systemincludes a cleaning vessel as generally described above, a let-downvalve in communication with the cleaning vessel outlet, a separator incommunication with the let-down valve, a condenser in communication withthe separator outlet for condensing the cleaning fluid to a liquidstate, and a storage vessel for maintaining the liquid fluid.

In operation, the liquid cleaning fluid is pumped from the storagevessel for cleaning operations and exposed to conditions of temperatureand pressure which cause the cleaning fluid to achieve a supercriticalstate. The cleaning fluid is then circulated in the cleaning vessel asdescribed, with a portion of the spent cleaning fluid removed from thecleaning vessel work zone at a predetermined rate when the let-downvalve is opened. The removed portion is passed to the separator where itis regenerated. The regenerated cleaning fluid thereafter passes throughthe separator outlet to the condenser and from there back to the liquidstorage vessel. The control means advantageously monitors and controlsflow rates and temperatures throughout the system to maintain a drivingpressures from the cleaning vessel to the storage vessel. The controlmeans also balances optimal solubility levels with the need to maintaina base system pressure below which the pressure in none of thecomponents falls so that commercially available condensing equipment maybe used.

In a preferred embodiment the system also includes a bypass extendingfrom a point downstream of the storage vessel to a second point upstreamof the condenser. The bypass includes a valve which may be controlled toadmit varying amounts of cleaning fluid into the bypass for controllingand pulsing the pressure within the cleaning vessel.

The following description, in conjunction with the accompanyingdrawings, discloses further details and advantages of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a supercritical cleaning system inaccordance with the present invention;

FIG. 2 is a cross section of a cleaning vessel in accordance with thepresent invention;

FIG. 3 is a plan view of the cleaning vessel of FIG. 2;

FIG. 4 is a plan view of an impeller and a heating element in thecleaning vessel of FIG. 2;

FIG. 5 is an isolation view of an entry filter arrangement for thecleaning vessel of FIG. 2;

FIG. 6 is a cross section of a separator in accordance with the presentinvention; and

FIG. 7 is a flow chart describing the process with which the apparatusof the present invention is utilized.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a system for supercritical cleaning in accordance with thepresent invention. Fig. 1 may be read in connection with Fig. 7 for anoverview of the flow path through the system. The cleaning fluid circuitbegins in a storage vessel 10 wherein the carbon dioxide is maintainedin a liquid state. When a cleaning cycle is initiated, the liquid carbondioxide is removed from storage vessel 10 by a liquid pump 12. Thecarbon dioxide is then passed through a preheater 14 where itstemperature is increased to above its supercritical point. From thepreheater 14, the carbon dioxide is then introduced through an inlet 15into a cleaning vessel 16, where it is pressurized and internallycirculated, as discussed below.

Spent carbon dioxide is removed from the cleaning vessel 16 via outlet18, and the rate of removal is controlled by let-down valve 20. Thecarbon dioxide passes through let-down valve 20 and enters a separator22, typically in a combined liquid and vapor phase. Flow through theseparator is regulated by let-down valve 24. The gas and liquid arefiltered, and liquid carbon dioxide and contaminants are distilled inthe separator as discussed below. Contaminant-free gaseous carbondioxide is passed to a condenser 26.

The system also includes a bypass 28 having a bypass valve 30 for shortcircuiting liquid carbon dioxide from the pump 12 directly to thecondenser 26 when a predetermined maximum pressure is present in thecleaning vessel 16. Appropriate pressure monitors 27 and temperaturemonitors 29 are located throughout the system, along with appropriatecontrollers 33.

Referring to FIGS. 2-4 and 6, the cleaning vessel 16 includes an inner,removable drum 32 having an impermeable body 34 with a removable entryfilter 36 at its upper end and a removable exit filter 38 at its lowerend. The drum is removably placed on a seat 40 inside the vessel 16, andthe vessel 16 and drum body 34 cooperate to define an annular space 42around the drum. The filters and the drum body cooperate to define asanitized work zone 45, as discussed below.

An impeller 44 is located beneath seat 40 as is an electric heater 46.The impeller is driven by an electric motor 47. Means (not shown), suchas a mesh basket, a series of racks or shelves or other arrangement, arelocated inside work one 45 for holding the parts to be cleaned.

The filters 36, 38 may be interchanged with filters of varying meshsizes, for example 40 microns, 30 microns, 20 microns, etc., to vary themesh size of particulates passable therethrough. To facilitateinterchangeability, tapped brackets 48 are located on a top 50 of drum32. The brackets 48 clamp entry filter 36 into position, utilizing athreaded bolt 49 or the like. The exit filter 38 rests in a recess 51 inseat 40.

The cleaning vessel 16 has a threaded upper lip 52, and a top cover 54rests on the upper lip. The top cover has a seal plate 56 whose outerdiameter corresponds to the inner diameter of vessel 16, along with aplurality of radially outward directed splines 58. Top cover 54 is alsoprovided with drilled taps 60 for receiving a lift device (not shown).

A top nut 62 is threadingly received on upper lip 52 and may be wounddown to clamp the top cover 54 and seal plate 56 onto the cleaningvessel 16 in a pressure resistant manner. The top nut includes aplurality of keyways 64 which receive splines 58 in the top cover 54.

In operation, preheated carbon dioxide is pumped into the cleaningvessel 16 via inlet 15 until a sufficient volume is present to maintainthe desired pressure. Temperature is controlled by heater 46, and atemperature and pressure controller 33 is preset to maintain the carbondioxide in a supercritical state while in the cleaning vessel 16. Carbondioxide is introduced from the inlet 15 to the annular space 42, and theimpeller 44 ensures circulation upward along body 34, and downwardthrough entry filter 36 into the sanitized work zone 45. The internalfiltration prevents redeposition of contaminants onto the parts oncecontaminants have been removed therefrom. The let-down valve 20 and pump12 are controlled to maintain pressure in cleaning vessel 16 throughoutthe cleaning cycle. In this regard, relatively small amounts of carbondioxide are removed from work zone 45 through outlet 18 when let-downvalve 20 is opened.

After the cleaning cycle is completed, the let-down valve is furtheropened to begin a pressure release from cleaning vessel 16.Particularly, the pressure release should be maintained at acontinuously varying rate to avoid carbon dioxide impregnation into theparts to be cleaned. Such impregnation could cause explosion orformation of gas bubbles in the parts. To facilitate this technique,several let-down valves 20 in parallel may be placed in communicationwith outlet 18, each opened in succession to maintain the desired flowrate and pressure release from cleaning vessel 16.

After passing through let-down valve 20, the cleaning fluid typicallychanges to a combined liquid and vapor phase. At this time it isintroduced to separator 22, shown in FIG. 5. The separator 22 includesan elongated body 72 with a concentric feed tube 74. The feed tube has aplurality of spaced apertures 76 adjacent its distal end and carries afirst particulate filter 78 adjacent the apertures 76. Although notshown in detail, the first filter 78 contains a plurality of foldedpleats having orifices which increase in size as they progress radiallyoutward on the filter. This arrangement provides a coalescing effect forparticulates in the cleaning fluid. The first filter is sealed at itsupper and lower ends so that all cleaning fluid issuing from the feedtube must pass through the first filter.

An inner capsule 80 surrounds first filter 78 and has an elongatedoutlet 82 at it,s lower end, while being sealed at its upper end. Theoutlet is disposed in a collection chamber 84 at the lower end ofseparator body 72. The collection chamber 84 may be evacuated through adrain 86. An internal heater 88 and a pair of thermocouples 90 aredisposed in the collection chamber.

A second de-misting filter 92 surrounds inner capsule 80 and occupies avapor passageway 94 between the capsule 80 and the inside of body 72.The second filter is constructed of compacted mesh wire for coalescingmist from uprising cleaning fluid vapors.

In operation, spent carbon dioxide passes through apertures 76 in feedtube 74 and enters first filter 78. Liquid carbon dioxide and coalescedcontaminants fall through outlet 82 of inner capsule 80 into collectionchamber 84. Thermocouples 90 detect temperatures to monitor the liquidlevel in collection chamber 84 so that controller 33 can operate heater88 to maintain the liquid level above outlet 82. Thus, pressurizedcleaning fluid in the gaseous state is bubbled up through the liquid inthe collection chamber to further remove particulates and mist from thegaseous fluid. The internal heater provides heat to liquid in thecollection chamber so that the liquid is essentially in a continuousboiling state, with carbon dioxide vapor emanating from the liquidsubstantially free of contaminants. The remaining contaminants andliquid are periodically evacuated through drain 86.

Utilizing control valve 24, and pressure monitor 27, the pressure in theseparator is controlled so that the carbon dioxide vapor exits theseparator at a pressure which maintains minimum contaminant solubilityin the carbon dioxide. The separator may be provided with insulation 98for more accurate heat control. Treated vapor exits through outlet 100at the upper end of the separator.

The control scheme employed for the cleaning system is as follows. Theinternal heaters of the preheater, cleaning vessel and separator vesselare controlled by independent Proportional, Integral and Derivative Term("PID") controllers 33. Maximum temperature set points in these vesselare preprogrammed and, if exceeded, the controller will disconnect powerto the appropriate heater. Flow rates in the system are controlled byvarying the number of strokes per minute of pump 12 using controller 33.Pump 12 is preferably a positive displacement liquid pump having aconstant volume per stroke.

Pressures in the cleaning vessel and separator vessel are independentlymaintained, while achieving maximum flow throughout the entire system.During the cleaning cycle, maximum system pressure is located in thecleaning vessel, with incrementally lower pressures found in theseparator vessel through the condenser into the storage vessel. A basesystem pressure, below which pressure in none of the components of thesystem may drop, is maintained in the storage vessel. This ensures thatcommercially available condensation equipment may be used in thecondenser. Lower pressures would necessitate more expensive,sophisticated equipment in order to condense the carbon dioxide to aliquid state. The incrementally stepped pressure distribution alsoensures that there is a driving force from the cleaning vessel throughthe system to the liquid storage vessel so that only one pump isrequired in the system for proper flow rates.

During the pressurization portion of the cleaning cycle, valves 20, 24and 30 remain closed until the cleaning vessel target pressure isreached. At that time, the flow mode is activated and valve 30 will openif the pressure in the cleaning vessel exceeds the target cleaningvessel pressure. This admits liquid carbon dioxide into the bypass toavoid further pressurization of the cleaning vessel. To minimize theamount of liquid carbon dioxide which must be bypassed, valve 20 isautomatically opened to release pressure in the cleaning vessel 16. Thecontrol system proportionally closes valve 30 while opening valve 20,and this valve action can be tuned so that 90-100% of the output of theliquid pump 12 is forced to go through the preheater 14 and into thecleaning vessel 16, as opposed to being bypassed.

When valve 20 is opened, carbon dioxide is forced into separator 22. Theresulting increase in the separator pressure will cause valve 24 to openif the following two conditions are met:

1) P_(separator) >P_(liquid) storage+25 psi ; and

2) P_(separator) >separator target pressure+"x" psi (where "x" isoperator adjustable).

Once a predetermined amount of carbon dioxide (operator setable) hasflowed through the system, the control system will initiate a let-downsequence which reduces pressure in the cleaning vessel. When thecleaning vessel pressure approaches a preset pressure higher than thebase pressure, the let-down sequence is defeated and a vent from thecleaning vessel directly to atmosphere is activated. The cleaning vessel16 may then be opened and the contaminant-free parts removed.Additionally, heater 46 is operated during let-down to warm the cleanedparts so that atmospheric moisture does not condense on the parts whenthe cleaning vessel is opened.

Having described the presently preferred embodiments of the invention,it will be understood that it is not intended to limit the inventionexcept within the scope of the following claims.

We claim:
 1. An apparatus for precision cleaning a workpiece with acleaning fluid comprising liquid carbon dioxide at temperatures andpressures which cause the cleaning fluid to achieve a supercriticalstate, said apparatus comprising:a pressure vessel, an open endedcleaning drum positioned within the pressure vessel for receiving theworkpiece, said drum having an extended body with an exit filteradjacent the lower end of said drum body, said drum body cooperatingwith said exit filter to define a work zone for receiving a workpiece,said pressure vessel and drum further defining an annular space aroundsaid drum body; a cleaning fluid comprising liquified carbon dioxidewithin the work zone such that the workpiece is submerged therein; aninlet; an outlet; said inlet opening into the pressure vessel incommunication with the annular space and said outlet drawing directlyfrom the work zone such that cleaning fluid newly introduced into thevessel is mixed with fluid that has been circulated around the drumprior to being withdrawn through the outlet; and means within thepressure vessel for circulating cleaning fluid around said drum downthrough said work zone and up along the annular space such that cleaningfluid introduced through said inlet may be directed along said drumbody, then through said work zone where it contacts the workpiece,finally passing through said exit filter and recirculating to theannular space, with a portion of the cleaning fluid drawn from saidoutlet.
 2. The apparatus of claim 1 wherein said drum body isimpermeable to the cleaning fluid.
 3. The apparatus of claim 1 furthercomprising means for removably interchanging said filters to vary themesh size of particulates passable therethrough.
 4. The apparatus ofclaim 1 further comprising a base on which said cleaning drum isremovably supported, said base defining a recessed filter seat forsupporting said exit filter.
 5. The apparatus of claim 1 furthercomprising a heating element for controlling the temperature within saidpressure vessel.
 6. The apparatus of claim 1 further comprising alet-down valve in communication with said outlet.
 7. A separator forremoving particulates and mist from a cleaning fluid in a system forcleansing a workpiece at supercritical conditions, comprising:anelongated body having an upper end and a lower end, said lower endhaving a drain; a feed tube having a proximal end and a distal endextending into said body, said feed tube having a plurality of aperturesadjacent its distal end; a tubular filter having an upper sealed end anda lower sealed end and being carried on the distal end of said feed tubeadjacent said apertures; a second filter carried on said feed tube atits proximal end; an outlet adjacent the upper end of said elongatedbody and in communication with said second filter; and a collectionchamber located adjacent the lower end of the body; such that thecleaning fluid passes through said feed tube and is introduced to saidfirst filter through the apertures, passes through said first filterwhere contaminants in the fluid are coalesced, then passes to thecollection chamber where cleaning fluid is vaporized then passes throughsaid second filter where mist is separated from the vaporized cleaningfluid, and exits the body through said outlet, with coalescedcontaminants and mist falling from the filters to the collection chamberto be removed through said drain.
 8. The separator of claim 7 furthercomprising a heating element for controlling the temperature within saidcollection chamber.
 9. The separator of claim 8 further comprising aninner capsule having a sealed upper end and an open lower end, saidcapsule enclosing the first filter and having a tubular outlet at itslower end, said capsule and elongated body defining an annular vaporpassageway between the capsule and the body, such that cleaning fluidpasses through the first filter and enters the capsule in a combinedliquid/vapor state, passing downward through the outlet into thecollection chamber where the liquid is collected and heated to distillcontaminants therefrom, vaporized cleaning fluid then passing upwardthrough said annular vapor passageway and through the second filter tothe separator outlet, nonevaporated contaminants removed through thedrain.
 10. The separator of claim 9 wherein said tubular outlet issufficiently long to extend below a liquid level in said collectionchamber so that vaporized cleaning fluid may be bubbled through theliquid.
 11. The separator of claim 7 further comprising at least onetemperature monitor for measuring temperatures within said collectionchamber.
 12. The separator of claim 7 further comprising a levelindicator device for maintaining a predetermined liquid level in saidcollection chamber.
 13. A system for cleaning a workpiece with asupercritical cleaning fluid, comprising:a pressure vessel, an openended cleaning drum positioned in the vessel and defining a work zonefor receiving the workpiece, said pressure vessel enclosing an annularspace around said drum, said pressure vessel having an inlet incommunication with said annular space and an outlet in communicationwith said work zone; a let-down valve in communication with said outlet;a heater for controlling the temperature of said cleaning fluid; aseparator in communication with said let-down valve, said separatorhaving a body with an inlet and an outlet and a contaminant drain at alower end of the separator; a condenser in communication with saidseparator outlet for condensing gaseous cleaning fluid to a liquidstate; a storage vessel for maintaining the liquid cleaning fluid; apump for conveying cleaning fluid from the storage vessel to thecleaning vessel; and control means for monitoring temperatures andpressures and operating said let-down valve, said heater and said pumpto maintain predetermined pressures, temperatures and flow ratesthroughout said system; such that liquid cleaning fluid may be pumpedfrom the storage vessel to the pressure vessel and caused by temperatureand pressure to achieve a supercritical state, with the cleaning fluidcirculated through the annular space into said work zone during acleaning cycle, said cleaning fluid removed from the work zone aftersaid cleaning cycle when said let-down valve is opened, said removedportion passed to the separator where it is regenerated, with thecleaning fluid thereafter passing through the separator outlet to thecondenser and from there back to the liquid storage vessel, withseparated contaminants collecting in the lower end of the separator forremoval through said drain.
 14. The system of claim 13 furthercomprising a bypass in communication with said pump and said condenser,said bypass having a valve which, when opened by said control means,admits cleaning fluid into said bypass to control the pressure withinsaid cleaning vessel.
 15. The system of claim 13 further comprising aplurality of let-down valves arranged in parallel for enhanced pressurereduction in said cleaning vessel after completion of said cleaningcycle.
 16. The system of claim 13 further comprising distillation meansin said separator for boiling liquid cleaning fluid and contaminants toliberate vaporized cleaning fluid, said separator having a filter incommunication with said Separator outlet, said vapor passing throughsaid filter and said outlet and from there to said condenser.
 17. Theapparatus of Claim 1 further comprising an entry filter on said Cleaningdrum, said entry filter cooperating with said cleaning drum and saidexit filter to define said work zone.